首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 640 毫秒
1.
Turbidity currents are turbulent, sediment‐laden gravity currents which can be generated in relatively shallow shelf settings and travel downslope before spreading out across deep‐water abyssal plains. Because of the natural stratification of the oceans and/or fresh water river inputs to the source area, the interstitial fluid within which the particles are suspended will often be less dense than the deep‐water ambient fluid. Consequently, a turbidity current may initially be denser than the ambient sea water and propagate as a ground‐hugging flow, but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient sea water. When this occurs, all or part of the turbidity current lofts to form a buoyant sediment‐laden cloud from which further deposition occurs. Deposition from such lofting turbidity currents, containing a mixture of fine and coarse sediment suspended in light interstitial fluid, is explored through analogue laboratory experiments complemented by theoretical analysis using a ‘box and cloud’ model. Particular attention is paid to the overall deposit geometry and to the distributions of fine and coarse material within the deposit. A range of beds can be deposited by bimodal lofting turbidity currents. Lofting may encourage the formation of tabular beds with a rapid pinch‐out rather than the gradually tapering beds more typical of waning turbidity currents. Lofting may also decouple the fates of the finer and coarser sediment: depending on the initial flow composition, the coarse fraction can be deposited prior to or during buoyancy reversal, while the fine fraction can be swept upwards and away by the lofting cloud. An important feature of the results is the non‐uniqueness of the deposit architecture: different initial current compositions can generate deposits with very similar bed profiles and grading characteristics, highlighting the difficulty of reconstructing the nature of the parent flow from field data. It is proposed that deposit emplacement by lofting turbidity currents is common in the geological record and may explain a range of features observed in deep‐water massive sands, thinly bedded turbidite sequences and linked debrites, depending on the parent flow and its subsequent development. For example, a lofting flow may lead to a well sorted, largely ungraded or weakly graded bed if the fines are transported away by the cloud. However, a poorly sorted, largely ungraded region may form if, during buoyancy reversal, high local concentrations and associated hindered settling effects develop at the base of the cloud.  相似文献   

2.
Flume experiments were performed to study the flow properties and depositional characteristics of high‐density turbidity currents that were depletive and quasi‐steady to waning for periods of several tens of seconds. Such currents may serve as an analogue for rapidly expanding flows at the mouth of submarine channels. The turbidity currents carried up to 35 vol.% of fine‐grained natural sand, very fine sand‐sized glass beads or coarse silt‐sized glass beads. Data analysis focused on: (1) depositional processes related to flow expansion; (2) geometry of sediment bodies generated by the depletive flows; (3) vertical and horizontal sequences of sedimentary structures within the sediment bodies; and (4) spatial trends in grain‐size distribution within the deposits. The experimental turbidity currents formed distinct fan‐shaped sediment bodies within a wide basin. Most fans consisted of a proximal channel‐levee system connected in the downstream direction to a lobe. This basic geometry was independent of flow density, flow velocity, flow volume and sediment type, in spite of the fact that the turbidity currents of relatively high density were different from those of relatively low density in that they exhibited two‐layer flow, with a low‐density turbulent layer moving on top of a dense layer with visibly suppressed large‐scale turbulence. Yet, the geometry of individual morphological elements appeared to relate closely to initial flow conditions and grain size of suspended sediment. Notably, the fans changed from circular to elongate, and lobe and levee thickness increased with increasing grain size and flow velocity. Erosion was confined to the proximal part of the leveed channel. Erosive capacity increased with increasing flow velocity, but appeared to be constant for turbidity currents of different grain size and similar density. Structureless sediment filled the channel during the waning stages of the turbidity currents laden with fine sand. The adjacent levee sands were laminated. The massive character of the channel fills is attributed to rapid settling of suspension load and associated suppression of tractional transport. Sediment bypassing prevailed in fan channels composed of very fine sand and coarse silt, because channel floors remained fully exposed until the end of the experiments. Lobe deposits, formed by the fine sand‐laden, high‐density turbidity currents, contained massive sand in the central part grading to plane parallel‐laminated sand towards the fringes. The depletive flows produced a radial decrease in mean grain size in the lobe deposits of all fans. Vertical trends in grain size comprised inverse‐to‐normal grading in the levees and in the thickest part of the lobes, and normal grading in the channel and fringes of the fine sandy fans. The inverse grading is attributed to a process involving headward‐directed transport of relatively fine‐grained and low‐concentrated fluid at the level of the velocity maximum of the turbidity current. The normal grading is inferred to denote the waning stage of turbidity‐current transport.  相似文献   

3.
Deep‐water sandstone beds of the Oligocene Fusaru Sandstone and Lower Dysodilic Shale, exposed in the Buz?u Valley area of the East Carpathian flysch belt, Romania, can be described in terms of the standard turbidite divisions. In addition, mud‐rich sand layers are common, both as parts of otherwise ‘normal’ sequences of turbidite divisions and as individual event beds. Eleven units, interpreted as the deposits of individual flows, were densely sampled, and 87 thin sections were point counted for grain size and mud content. S3/Ta divisions, which form the bulk of most sedimentation units, have low internal textural variability but show subtle vertical trends in grain size. Most commonly, coarse‐tail normal grading is associated with fine‐tail inverse grading. The mean grain size can show inverse grading, normal grading or a lack of grading, but sorting tends to improve upward in most beds. Fine‐tail inverse grading is interpreted as resulting from a decreasing effectiveness of trapping of fines during rapid deposition from a turbidity current as the initially high suspended‐load fallout rate declines. If this effect is strong enough, the mean grain size can show subtle inverse grading as well. Thus, thick inversely graded intervals in deep‐water sands lacking traction structures do not necessarily imply waxing flow velocities. If the suspended‐load fallout rate drops to zero after the deposition of the coarse grain‐size populations, the remaining finer grained flow bypasses and may rework the top of the S3 division, forming well‐sorted, coarser grained, current‐structured Tt units. Alternatively, the suspended‐load fallout rate may remain high enough to prevent segregation of fines, leading to the deposition of significant amounts of mud along with the sand. Mud content of the sandstones is bimodal: either 3–13% or more than 20%. Two types of mud‐rich sandstones were observed. Coarser grained mud‐rich sandstones occur towards the upper parts of S3/Ta divisions. These units were deposited as a result of enhanced trapping of mud particles in the rapidly deposited sediment. Finer grained mud‐rich units are interbedded with ripple‐laminated very fine‐grained sandy Tc divisions. During deposition of these units, mud floccules were hydraulically equivalent to the very fine sand‐ and silt‐sized sediment. The mud‐rich sandstones were probably deposited by flows that became transitional between turbidity currents and debris flows during their late‐stage evolution.  相似文献   

4.
Turbidity currents in the ocean are driven by suspended sediment. Yet results from surveys of the modern sea floor and turbidite outcrops indicate that they are capable of transporting as bedload and depositing particles as coarse as cobble sizes. While bedload cannot drive turbidity currents, it can strongly influence the nature of the deposits they emplace. This paper reports on the first set of experiments which focus on bedload transport of granular material by density underflows. These underflows include saline density flows, hybrid saline/turbidity currents and a pure turbidity current. The use of dissolved salt is a surrogate for suspended mud which is so fine that it does not settle out readily. Thus, all the currents can be considered to be model turbidity currents. The data cover four bed conditions: plane bed, dunes, upstream‐migrating antidunes and downstream‐migrating antidunes. The bedload transport relation obtained from the data is very similar to those obtained for open‐channel flows and, in fact, is fitted well by an existing relation determined for open‐channel flows. In the case of dunes and downstream‐migrating antidunes, for which flow separation on the lee sides was observed, form drag falls in a range that is similar to that due to dunes in sand‐bed rivers. This form drag can be removed from the total bed shear stress using an existing relation developed for rivers. Once this form drag is subtracted, the bedload data for these cases collapse to follow the same relation as for plane beds and upstream‐migrating antidunes, for which no flow separation was observed. A relation for flow resistance developed for open‐channel flows agrees well with the data when adapted to density underflows. Comparison of the data with a regime diagram for field‐scale sand‐bed rivers at bankfull flow and field‐scale measurements of turbidity currents at Monterey Submarine Canyon, together with Shields number and densimetric Froude number similarity analyses, provide strong evidence that the experimental relations apply at field scale as well.  相似文献   

5.
MATRIX OF TURBIDITES: EXPERIMENTAL APPROACH   总被引：2，自引：0，他引：2  
The matrix (< 40 μ) of turbidites forms a possible clue to the density of turbidity currents and the origin of the graywacke matrix. Experiments in a circular flume provide a mechanism to study the relation between composition of suspensions at various speeds and their deposits. There is a close analogy to the lower part of turbidity currents. The lutum content of samples with median diameters greater than 400 or 500 μ is found to correspond to the suspended load of the pore water. The higher value for finer deposits can be recalculated to suspension concentration by use of the “sedimentation factor”. Hence, each turbidite carries, as it were, a sample of its depositing current. The lutum content depends not on the ratio of sand to lutum in the current, as tacitly assumed by many authors, but mainly on the ratio lutum to water, although also influenced by velocity. The average lutum density of coarser recent deep-sea sands is 1-2%. This indicates turbidity currents with 5-10% lutum by weight (density 1.03–1.07). The sand must be added to ascertain the current density. In first approximation turbidity currents tend to have densities at their nose of 1.1–1.2, but higher and much lower values also occur. The maximum original lutum percentage of coarse turbidites is below 10%. Higher values are very scarce and are due to post-depositional mixing, or we are dealing with slides. However, in fine-grained turbidites there is more matrix up to 20% for a median of 100 p. Hence, coarse graded marine graywackes with 20 or more per cent matrix are presumably weakly metamorphic turbidites, that originally held the same modest amount of lutum as recent turbidites of the same grain size. The Trask sorting of the experimental deposits is very good, like the average of natural turbidites. Most cumulative curves of turbidite grain-size analyses on arithmetic probability paper show a characteristic bend in fine sand or silt sizes.  相似文献   

6.
A process-based, forward computer model of turbidity current flow and sedimentation, termed the TCFS model, has been developed to trace the downslope evolution of individual turbidity flows. Details of the model itself have been presented in a preceding paper. We here outline a series of tests of the TGFS model. The sensitivity tests of the TCFS model to general geological controls reveal the quantitative relationship between these controls and the behaviour of turbidity flows and the geometry and textural features of the resulting turbidites. Experimental turbidity currents on relatively steep slopes accelerate more rapidly and reach higher velocities than those on gentle slopes. Flows with larger initial volumes have higher initial velocities, travel further downslope, and form beds of greater thickness and downslope extent than smaller flows. Experimental high-concentration flows with suspended-sediment concentrations of 25% accelerate more rapidly and reach higher downslope velocities than dilute flows with 5% suspended sediment. The higher velocities and enhanced hindered-settling effects of the high-concentration flows lead to much greater transport distances and reduced vertical and lateral sediment size grading in the resulting turbidites. Beds formed by experimental high-concentration flows are massive or show coarse-tail grading whereas beds formed by low-concentration flows show distribution-grading. Experimental flows fed by coarse sediment sources tend to deposit the bulk of their suspended sediment loads on the proximal slope, resulting in more rapid flow deceleration and sedimentation than flows fed by silt-rich, fine-grained sediment sources. Turbidites formed by coarse-sediment flows tend to have a wedge-shaped geometry, with low downslope extent and high surface relief, whereas turbidites formed by fine-sediment flows tend to have a tabular geometry, with greater downslope extent and lower surface relief. A specific geological test of the TCFS model is based on studies of modern turbidity currents in Bute Inlet, British Columbia, Canada. With the input initial and boundary conditions estimated from Bute Inlet, the model predicts the downslope velocity evolution of turbidity currents comparable to those of modern and ancient turbidity flows measured in Bute Inlet. Model-calculated vertical and downslope grain-size properties of turbidites are similar to those exhibited by surface and cored Bute Inlet turbidites. Model flows tend to decelerate more rapidly than some stronger turbidity currents in the Bute Inlet system, and model beds tend to decrease in grain-size downslope more rapidly than observed bottom sediments. This is probably because the TCFS model flows lacked clay, which is abundant in Bute Inlet; they do not fully simulate turbulent mixing of suspended sediments; and they better represent the unsteady, depositional stage of turbidity-currents than the preceding stage of more-or-less steady-flow conditions. These tests demonstrate that the TCFS model provides a semi-quantitative method to study the growth patterns of submarine turbidite systems. It can serve as a predictive tool for analysing the facies architecture of ancient turbidite systems through simulating multi-depositional events by improving its erosion function, and the compatibility between its numerical components.  相似文献   

7.
In the classical view of fine sediment transport and deposition in streams, particles are expected to be removed from flowing water simply by direct sedimentation onto the streambed. However, recent research has demonstrated that fine sediments can propagate into pore spaces in the streambed due to hyporheic exchange and be removed by a combination of physical and chemical processes. This behaviour can significantly alter fine sediment size distributions during in-stream sediment transport because the physical transport of fine particles and their attachment to bed sediment grains are both a function of the particle size. Herein, we present model simulations for deposition of suspended sediments with a bimodal size distribution. We also applied this approach to analyse the results of laboratory flume observations of suspended sediment deposition. Results from model simulations and flume experiments clearly show that the rate of particle deposition increases with increasing particle size. Thus, the larger particles are preferentially removed from mixtures and there is a fining of the mixed suspensions over time. Both particle deposition mechanisms, i.e. particle sedimentation and filtration, contribute to the fining of the mixed fine particle suspensions over time, and their effects are clearly demonstrated using the fundamental process-based model. These results clearly demonstrate the effects of stream-subsurface exchange on the temporal evolution of the suspended fine sediment size distribution in downstream transport.  相似文献   

8.
A three-dimensional, intratidal sediment transport model is developed for the estuarine turbidity maximum (ETM) in the upper Chesapeake Bay. The model considers three particle size classes, including the fine class mostly in suspension in the water column, the medium class alternately suspended and deposited by tidal currents, and the coarse size suspended only during the times of relatively high energy events. Based on the results of a box model, depth-limited erosion with continuous deposition is employed for the medium and coarse classes by varying the critical shear stress for erosion as a function of eroded mass. For the fine class, mutually exclusive erosion and deposition is employed with a small constant value for the critical shear stresses for erosion and deposition to assure quick erosion of recently deposited fine particles but without allowing further erosion of consolidated bed sediments. The model is run to simulate the annual condition in 1996, and the model generally gives a reasonable reproduction of the observed characteristics of the ETM relative to the salt limit and tidal phase. The model results for 1996 are analyzed to study the characteristics of the ETM along the main channel of the upper bay in intertidal and intratidal time scales. Under a low flow condition, local erosion/deposition and bottom horizontal flux convergence are the main processes responsible for the formation of the ETM, with the settling flux confining the ETM to the bottom water. Under a high flow condition, a distinctive ETM is formed by strong convergence of the downstream flux of sediments eroded from the upstream of the null zone and the upstream flux of sediments settled at the downstream of the null zone. Intratidal variation of the ETM is mainly controlled by erosion and the tidal transport of eroded sediments for a low flow condition. Under the direct influence of a high flow event, the ETM is mainly formed by erosion during ebbing tidal current strengthened by large freshwater discharge and by convergence of ebbing freshwater discharge and flooding tidal current. During the rebounding stage of a high flow event, intratidal variations are mainly controlled by tidal asymmetry caused by the interaction between tidal currents, gravitational circulation, and stratification.  相似文献   

9.
A computer-based numerical model of turbidity current flow and sedimentation is presented that integrates geological observations with basic equations for fluid and sediment motion. The model quantifies those aspects of turbidity currents that make them different from better-understood fluvial processes, including water mixing across the upper flow boundary and the interactions between the suspended-sediment concentration and the flow dynamics and sedimentation. The model includes three numerical components: (1) a layer-averaged three-equation flow model for tracing downslope flow evolution using continuity and momentum equations, (2) a sedimentation/fluidization model for tracing sediment-size fractionation in sedimenting multicomponent suspensions and (3) a concentration-viscosity model for quantifying the changes in resistance of such suspensions toward fluid and sediment motion. The model traces the evolution of a model turbidity current in terms the layer-averaged flow velocity, flow thickness, sediment concentration distribution, and the rate of sedimentation and sediment size fractionation. It generates synthetic turbidites with downslope variations in thickness and grain-size structuring at each point along the flow path. This study represents an effort to evaluate quantitatively the effects of basin geometry, sediment supply and sediment properties on the mechanics of turbidity current flow and sedimentation and on the geometry and grain size characteristics of the resulting deposits.  相似文献   

10.
《Sedimentology》2018,65(3):931-951
Submarine leveed channels are sculpted by turbidity currents that are commonly highly stratified. Both the concentration and the grain size decrease upward in the flow, and this is a fundamental factor that affects the location and grain size of deposits around a channel. This study presents laboratory experiments that link the morphological evolution of a progressively developing leveed channel to the suspended sediment structure of the turbidity currents. Previously, it was difficult to link turbidity current structure to channel–levee development because observations from natural systems were limited to the depositional products while experiments did not show realistic morphodynamics due to scaling issues related to the sediment transport. This study uses a novel experimental approach to overcome scaling issues, which results in channel inception and evolution on an initially featureless slope. Depth of the channel increased continuously as a result of levee aggradation combined with varying rates of channel floor aggradation and degradation. The resulting levees are fining upward and the grain‐size trend in the levee matches the upward decrease in grain size in the flow. It is shown that such deposit trends can result from internal channel dynamics and do not have to reflect upstream forcing. The suspended sediment structure can also be linked to the lateral transition from sediment bypass in the channel thalweg to sediment deposition on the levees. The transition occurs because the sediment concentration is below the flow capacity in the channel thalweg, while higher up on the channel walls the concentration exceeds capacity resulting in deposition of the inner levee. Thus, a framework is provided to predict the growth pattern and facies of a levee from the suspended sediment structure in a turbidity current.  相似文献   

11.
Turbidity currents, initiated from spring runoffs of an influent river, were observed in the upper region of a reservoir in Hokkaido, Japan, by measuring water temperature, velocity and suspended-sediment concentration. Their profiles offer some physical parameters for the sedimentary conditions, assuming the turbidity currents to be quasi-uniform. The bottom sediment deposited by the turbidity currents was then collected by a portable core sampler. The bottom sediment consists of more than 90% silt and clay, and thus offers a hydraulically smooth bed for shear flow; a plane bed as a bed configuration was formed on the reservoir bed, probably because of the low shear velocity and small grain size of sediment. Using a graphic method with log-normal probability paper, the bottom sediment is divided into several overlapping log-normal subpopulations. Grain-size analysis indicates that the bottom sediment may be regarded as cohesionless; criteria for ‘complete deposition’ of transported grains can then be incorporated into the ‘extended Shields diagram’ giving the minimum shear stress to erode bottom sediment. Applying the new diagram to the grain size distribution of the bottom sediment, it is suggested that each of the log-normal subpopulations was deposited in each of four different ‘modes of deposition’, i.e. ‘traction’, ‘saltation (or intermittent suspension)’, ‘suspension’ and ‘suspension under equilibrium’. The last mode may be observed under a sedimentary condition where upward flux of suspended sediment by eddy diffusion is almost equal to its depositional flux due to gravity. The mean and critical grain sizes for bottom sediment and each of the corresponding subpopulations decrease consistently with an increase of Ψ=Fd2 log10Re (Fd is the densimetric Froude number and Re is the flow Reynolds number). Ψ correlates inversely with shear velocity, which bears a linear relationship to mean velocity. These results lead to the conclusion that relatively fine suspended sediment is deposited as a result of decreasing bottom friction with a relative decrease of turbulent energy.  相似文献   

12.
Status and Trends in Research on Deep-Water Gravity Flow Deposits   总被引：3，自引：0，他引：3  
Deep-water gravity flows are one of the most important sediment transport mechanisms on Earth. After 60 years of study,significant achievements have been made in terms of classification schemes,genetic mechanisms,and depositional models of deep-water gravity flows. The research history of deep-water gravity flows can be divided into five stages: incipience of turbidity current theory; formation of turbidity current theory; development of deep-water gravity flow theory; improvement and perfection of deep-water gravity flow theory; and comprehensive development of deep-water gravity flow theory. Currently,three primary classification schemes based on the sediment support mechanism,the rheology and transportation process,and the integration of sediment support mechanisms,rheology,sedimentary characteristics,and flow state are commonly used.Different types of deep-water gravity flow events form different types of gravity flow deposits. Sediment slump retransportation mainly forms muddy debris flows,sandy debris flows,and surge-like turbidity currents. Resuspension of deposits by storms leads to quasi-steady hyperpycnal turbidity currents(hyperpycnal flows). Sustainable sediment supplies mainly generate muddy debris flows,sandy debris flows,and hyperpycnal flows. Deep-water fans,which are commonly controlled by debris flows and hyperpycnal flows,are triggered by sustainable sediment supply; in contrast,deep-water slope sedimentary deposits consist mainly of debris flows that are triggered by the retransportation of sediment slumps and deep-water fine-grained sedimentary deposits are derived primarily from finegrained hyperpycnal flows that are triggered by the resuspension of storm deposits. Harmonization of classification schemes,transformation between different types of gravity flow deposit,and monitoring and reproduction of the sedimentary processes of deep-water gravity flows as well as a source-to-sink approach to document the evolution and deposition of deep-water gravity flows are the most important research aspects for future studies of deep-water gravity flows study in the future.  相似文献   

13.
Sedimentation from gravity currents generated by turbulent plumes   总被引：4，自引：0，他引：4  
Sedimentation from radially spreading gravity currents generated at the top of ascending sediment-laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current, resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current. For radial spreading this model predicts a Gaussian distribution of sediment accumulation away from the source with an exponential constant, B, which depends on flow rate, Q, and particle settling velocity, v (B=nv/Q). In the experiments described, sedimentation occurs from gravity currents generated by ascent of buoyant, particle-laden plumes of fresh water in a tank of salty water. The sediment accumulation shows close agreement with the theoretical model, and the Gaussian decay constant, B, can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line In(S/S0) = -Br2, where r is the radial distance, S is the sediment mass flux per unit area and S0 is the value of S at r=0. Data from the dispersal of volcanic ejecta from a large (c. 24 km high) plinian eruption column in the Azores also show good agreement with the theory, confirming that it is general and independent of scale and the nature of the fluid. The experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity. Sedimentation of coarse grain sizes, between the source and the corner, occurs from the inclined plume margins and does not behave as predicted by the theoretical model.  相似文献   

14.
Carbonate environments inhabit the realm of the surface, intermediate and deep currents of the ocean circulation where they produce and continuously deliver material which is potentially deposited into contourite drifts. In the tropical realm, fine‐grained particles produced in shallow water and transported off‐bank by tidal, wind‐driven, and cascading density currents are a major source for transport and deposition by currents. Sediment production is especially high during interglacial times when sea level is high and is greatly reduced during glacial times of sea‐level lowstands. Reduced sedimentation on carbonate contourite drifts leads to early marine cementation and hardened surfaces, which are often reworked when current strength increases. As a result, reworked lithoclasts are a common component in carbonate drifts. In areas of temperate and cool water carbonates, currents are able to flow across carbonate producing areas and incorporate sediment directly to the current. The entrained skeletal carbonate particles have variable bulk density and shapes that lower the prediction of transport rates in energy‐based transport models, as well as prediction of current velocity based on grain size. All types of contourite drifts known in clastic environments are found in carbonate environments, but three additional drift types occur in carbonates because of local sources and current flow diversion in the complicated topography inherent to carbonate systems. The periplatform drift is a carbonate‐specific plastered drift that is nearly exclusively made of periplatform ooze. Its geometry is built by the interaction of along‐slope currents and downslope currents, which deliver sediment from the adjacent shallow‐water carbonate realm to the contour current via a line source. Because the periplatform drift is plastered on the slopes of the platforms it is also subject to mass gravity flow and large slope failures. At platform edges, a special type of patch drift develops. These hemiconal platform‐edge drifts also contain exclusively periplatform ooze but their geometry is controlled by the current around the corner of the platform. At the north‐western end of Little and Great Bahama Bank are platform‐edge drifts that are over 100 km long and up to 600 m thick. A special type of channel‐related drift forms when passages between carbonate buildups or channels within a platform open into deeper water. A current flowing in these channels will entrain material shed from the sediment producing areas. At the channel mouth, the sediment‐charged current deposits its sediment load into the deeper basin. With continuous flow, a submarine delta drift is built that progrades into the deep water. The strongly focused current forming the delta drift, is able to rework coarse skeletal grains and clasts, making this type of carbonate drift the coarsest drift type.  相似文献   

15.
The behaviour of subaerial particle-laden gravity currents (e.g. pyroclastic flows, lahars, debris flows, sediment-bearing floods and jökulhlaups) flowing into the sea has been simulated with analogue experiments. Flows of either saline solution, simple suspensions of silicon carbide (SiC) in water or complex suspensions of SiC and plastic particles in methanol were released down a slope into a tank of water. The excess momentum between subaerial and subaqueous flow is dissipated by a surface wave. At relatively low density contrasts between the tank water and the saline or simple suspensions, the flow mixture enters the water and forms a turbulent cloud involving extensive entrainment of water. The cloud then collapses gravitationally to form an underwater gravity current, which progresses along the tank floor. At higher density contrasts, the subaerial flow develops directly into a subaqueous flow. The flow slows and thickens in response to the reduced density contrast, which is driving motion, and then continues in the typical gravity current manner. Complex suspensions become dense flows along the tank floor or buoyant flows along the water surface, if the mixtures are sufficiently denser or lighter than water respectively. Flows of initially intermediate density are strongly influenced by the internal stratification of the subaerial flow. Material from the particulate-depleted upper sections of the subaerial flow becomes a buoyant gravity current along the water surface, whereas material from the particulate-enriched lower sections forms a dense flow along the tank floor. Sedimentation from the dense flow results in a reduction in bulk density until the mixture attains buoyancy, lifts off and becomes a secondary buoyant flow along the water surface. Jökulhlaups, lahars and debris flows are typically much denser than seawater and, thus, will usually form dense flows along the seabed. After sufficient sedimentation, the freshwater particulate mixture can lift off to form a buoyant flow at the sea surface, leading to a decoupling of the fine and coarse particles. Flood waters with low particulate concentrations (<2%) may form buoyant flows immediately upon entering the ocean. Subaerial pyroclastic flows develop a pronounced internal stratification during subaerial run-out and, thus, a flow-splitting behaviour is probable, which agrees with evidence for sea surface and underwater flows from historic eruptions of Krakatau and Mont Pelée. A pyroclastic flow with a bulk density closer to that of sea water may form a turbulent cloud, resulting in the deposition of much of the pyroclasts close to the shore. Dense subaqueous pyroclastic flows will eventually lift off and form secondary buoyant flows, either before or after the transformation to a water-supported nature.  相似文献   

16.
A laboratory flume experiment was carried out in which the hydrodynamic and sedimentary behaviour of a turbidity current was measured as it passed through an array of vertical rigid cylinders. The cylinders were intended primarily to simulate aquatic vegetation canopies, but could equally be taken to represent other arrays of obstacles, for example forests or offshore wind turbines. The turbidity currents were generated by mixing naturally sourced, poly‐disperse sediment into a reservoir of water at concentrations from 1·0 to 10·0 g l?1, which was then released into the experimental section of the flume by removing a lock gate. For each initial sediment concentration, runs with obstacle arrays with solid plant fractions of 1·0% and 2·5%, and control cases with no obstacles, were carried out. The progress of the current along the flume was characterized by the array drag term, CDaxc (where CD is the array drag coefficient, at the frontal area of cylinders per unit volume, and xc is the position of the leading edge of the current along the flume). The downward depositional flux of sediment out of the current as it proceeded was measured at 13 traps along the flume. Analysis of these deposits divided them into fine (2·2 to 6·2 μm) and coarse (6·2 to 104 μm) fractions. At the beginning of their development, the gravity currents proceeded in an inertia‐dominated regime until CDaxc = 5. For CDaxc > 5, the current transitioned into a drag‐dominated regime. For both fine and coarse sediment fractions, the rate of sediment deposition tended to decrease gradually with distance from the source in the inertial regime, remained approximately constant at the early drag‐dominated regime, and then rose and peaked at the end of the drag‐dominated stage. This implies that, when passing through arrays of obstacles, the turbidity currents were able to retain sufficient sediment in suspension to maintain their flow until they became significantly influenced by the drag exerted by the obstacles.  相似文献   

17.
Trapping of sustained turbidity currents by intraslope minibasins   总被引：1，自引：0，他引：1  
Depositional turbidity currents have filled many intraslope minibasins with sediment creating targets for petroleum exploration. The dynamics of sustained turbidity currents and their depositional characteristics are investigated in a scaled physical model of a minibasin. Each turbidity current deposited a downstream thinning wedge of sediment near the inlet. Farther downstream the turbidity current was ponded by a barrier. The ponded part of the turbidity current was separated from the sediment‐free water above by a relatively sharp, horizontal settling interface indicating highly Froude‐subcritical flow. The very slow moving flow within the ponded zone created conditions for the passive rainout of suspended sediment onto the bed. In the lower part of the ponded zone, the concentration and mean grain‐size of the sediment in suspension tended to be relatively uniform in both the vertical and streamwise directions. As a result, the deposit emplaced in the ponded zone showed only a weak tendency toward downstream fining and was passively draped over the bed in such a way that irregularities in the inerodible bed were accurately reflected. The discharge of suspended sediment overflowing the downstream end of the minibasin was significantly less than the inflow discharge, resulting in basin sediment trapping efficiencies >95%. A simple model is developed to predict the trapping of sediment within the basin based on the relative magnitudes of the input discharge of turbid water and the detrainment discharge of water across the settling interface. This model shows a limiting case in which an intraslope basin captures 100% of the sediment from a ponded turbidity current, even through a succession of sustained flow events, until sediment deposition raises the settling interface above the downstream lip of the minibasin. This same process defines one of the mechanisms for minibasin filling in nature, and, when this mechanism is operative, the trap efficiency of sediment can be expected to be high until the minibasin is substantially filled with sediment.  相似文献   

18.
Abstract

The lithology and primary structural features of the Ordovician rocks of Victoria, and also of many of the Silurian rocks, are explicable according to the hypothesis of turbidity currents, but it is necessary to account for the preservation of a very complete succession of graptolite-bearing horizons in these rocks. It is suggested that the graptolites which are fossilized within graded beds, occurring towards the top in their silty or clayey portions, represent communities killed by fine suspended detritus in the upper parts of turbidity currents. Each graded bed is thus taken to represent a very small portion of time, and the duration of the diastems on major bedding planes is, correspondingly, much greater. A review of sedimentation and vulcanicity within the Victorian geosynclinal zone in the lower Palaeozoic is given.  相似文献   

19.
M. Felix 《Sedimentology》2002,49(3):397-419
A two‐dimensional numerical model is used to describe the flow structure of turbidity currents in a vertical plane. To test the accuracy of the model, it is applied to historical flows in Bute Inlet and the Grand Banks flow. The two‐dimensional spatial and temporal distributions of velocity and sediment concentration and non‐dimensionalized vertical profiles of velocity, turbulent kinetic energy and sediment concentration are discussed for several simple computational currents. The flows show a clear interaction between velocity, turbulence and sediment distribution. The results of the numerical tests show that flows with fine‐grained sediment have low vertical and high horizontal gradients of velocity and sediment concentration, show little increase in flow thickness and decelerate slowly. Steadiness and uniformity in these flows are comparable for velocity and concentration. In contrast, flows with coarse‐grained sediment have high vertical and low horizontal velocity gradients and high horizontal concentration gradients. These flows grow considerably in thickness and decelerate rapidly. Steadiness and uniformity in flows with coarse‐grained sediment are different for velocity and concentration. The results show the influence of spatial and temporal flow structure on flow duration and sediment transport.  相似文献   

20.
流体性质转换机制在重力流沉积体系分析中应用初探   总被引：5，自引：0，他引：5  
自1950年Kuenen发表"浊流形成粒序层理"一文至今,深水重力流研究取得了长足发展。然而,目前很多学者不仅对浊流理论分歧很大,而且对浊流概念、术语的使用也有不同的观点,这严重制约了重力流的分类及重力流沉积体系的识别。本文在重力流沉积过程和流态研究的基础上,提出了基于"流体性质转换"理论的重力流沉积体系分析方法。研究认为,重力流在形成、发展和消亡过程中不仅存在多个流体阶段,而且存在着多种流态之间的转换。这种流体发育的最终阶段特征决定了重力流沉积体系的形态与类别,而流态共存与转换的特性造成了深水紊流成因的浊积岩与其他流体成因的岩相共存的沉积特征。根据这一理论对牵引流—碎屑流成因的扇三角洲体系、浊流成因的近岸水下扇体系和碎屑流成因的斜坡裙体系进行成因分析与特征对比,阐述了"流体性质转换"理论在沉积体系识别中的作用。  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号